1. Field of the Invention
This invention relates to the field of cobalt-base alloys. Such alloys find particular use in applications such as gas turbine engines where oxidation and corrosion at elevated temperatures are problems. The present alloys may be used in cast or wrought articles and may be used uncoated in many applications.
2. Description of the Prior Art
The increasing demands for performance and efficiency which have been placed on gas turbines have been largely met by employing higher operating temperatures. These higher operating temperatures require improved materials with resistance to oxidation and corrosion at elevated temperatures in combination with good mechanical properties.
While many high temperature parts of complex geometry are produced by casting, other parts of thin and uniform cross section may most satisfactorily be produced by hot and cold working techniques. Such wrought materials are especially important where weight must be minimized.
In high temperature alloys, inherent oxidation resistance usually results from an oxide layer which forms in service. Oxidation resistance will be improved if the oxide layer can be prevented from spalling off the surface during thermal cycling. In particularly demanding environments, many alloys need further protection to provide an adequate service life. This further protection may be provided by coatings.
In most current cobalt base alloys, the oxide layer which forms is based on chromium (Cr) and generally the oxidation resistance is not sufficient to permit uncoated operation in demanding environments. Aluminum (Al) is not a common alloying addition to commercial cobalt alloys since in most cobalt alloys, the amount of Al required to provide a protective alumina layer during the life of the part is excessive and can cause problems with mechanical properties and fabricability. The excessive amount of Al required is related to the spallation of the alumina which requires that sufficient Al be present to repeatedly reform the alumina layer. This process eventually depletes the underlying alloy in Al, leading to rapid oxidation.
ASTM Special Technical Publication No. 170-A, by W. F. Simmons and V. N. Kribivobok, "Compilation of Chemical Compositions and Rupture Strengths of Super-Strength Alloys", discloses only two cobalt superalloys which contain Al, alloy M 205 which contains 2.75% Al, and alloy M 203 which contains 0.75% Al. Cobalt base alloys AR 213 and AR 215 which contain about 4% Al have been introduced but are not widely used.
Yttrium (Y) has been found to improve the oxidation resistance of certain nickel base superalloys, see for example, U.S. Pat. No. 3,202,506 which discloses the addition of Y to nickel (Ni) alloys. Coating compositions containing Y and Al in a cobalt base are known in the art, see for example U.S. Pat. No. 3,676,085 which is assigned to the present assignee. Such coating compositions are invariably brittle, because of high Y and Al levels, and have relatively low strengths. U.S. Pat. No. 3,399,058 discloses a cobalt base alloy which may contain Y greatly in excess of the solid solubility limit with the result that it contains excessive amounts of brittle, low melting phases and therefore has inferior mechanical properties and fabricability. A similar use of Y in combination with Al is found in U.S. Pat. No. 3,027,252, however the alloy disclosed has an iron (Fe) base. Belgian Pat. No. 766,596 also discloses a cobalt alloy containing Y and Al.
Hafnium (Hf) has previously been used in certain nickel base alloys, as described for example in U.S. Pat. No. 3,005,705, for the purpose of improving elevated temperature ductility but is not a common addition to cobalt base alloys.